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<span id="Values-in-Registers"></span><div class="header">
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<span id="How-Values-Fit-in-Registers"></span><h4 class="subsection">18.7.3 How Values Fit in Registers</h4>

<p>This section discusses the macros that describe which kinds of values
(specifically, which machine modes) each register can hold, and how many
consecutive registers are needed for a given mode.
</p>
<dl>
<dt id="index-TARGET_005fHARD_005fREGNO_005fNREGS">Target Hook: <em>unsigned int</em> <strong>TARGET_HARD_REGNO_NREGS</strong> <em>(unsigned int <var>regno</var>, machine_mode <var>mode</var>)</em></dt>
<dd><p>This hook returns the number of consecutive hard registers, starting
at register number <var>regno</var>, required to hold a value of mode
<var>mode</var>.  This hook must never return zero, even if a register
cannot hold the requested mode - indicate that with
<code>TARGET_HARD_REGNO_MODE_OK</code> and/or
<code>TARGET_CAN_CHANGE_MODE_CLASS</code> instead.
</p>
<p>The default definition returns the number of words in <var>mode</var>.
</p></dd></dl>

<dl>
<dt id="index-HARD_005fREGNO_005fNREGS_005fHAS_005fPADDING">Macro: <strong>HARD_REGNO_NREGS_HAS_PADDING</strong> <em>(<var>regno</var>, <var>mode</var>)</em></dt>
<dd><p>A C expression that is nonzero if a value of mode <var>mode</var>, stored
in memory, ends with padding that causes it to take up more space than
in registers starting at register number <var>regno</var> (as determined by
multiplying GCC&rsquo;s notion of the size of the register when containing
this mode by the number of registers returned by
<code>TARGET_HARD_REGNO_NREGS</code>).  By default this is zero.
</p>
<p>For example, if a floating-point value is stored in three 32-bit
registers but takes up 128 bits in memory, then this would be
nonzero.
</p>
<p>This macros only needs to be defined if there are cases where
<code>subreg_get_info</code>
would otherwise wrongly determine that a <code>subreg</code> can be
represented by an offset to the register number, when in fact such a
<code>subreg</code> would contain some of the padding not stored in
registers and so not be representable.
</p></dd></dl>

<dl>
<dt id="index-HARD_005fREGNO_005fNREGS_005fWITH_005fPADDING">Macro: <strong>HARD_REGNO_NREGS_WITH_PADDING</strong> <em>(<var>regno</var>, <var>mode</var>)</em></dt>
<dd><p>For values of <var>regno</var> and <var>mode</var> for which
<code>HARD_REGNO_NREGS_HAS_PADDING</code> returns nonzero, a C expression
returning the greater number of registers required to hold the value
including any padding.  In the example above, the value would be four.
</p></dd></dl>

<dl>
<dt id="index-REGMODE_005fNATURAL_005fSIZE-2">Macro: <strong>REGMODE_NATURAL_SIZE</strong> <em>(<var>mode</var>)</em></dt>
<dd><p>Define this macro if the natural size of registers that hold values
of mode <var>mode</var> is not the word size.  It is a C expression that
should give the natural size in bytes for the specified mode.  It is
used by the register allocator to try to optimize its results.  This
happens for example on SPARC 64-bit where the natural size of
floating-point registers is still 32-bit.
</p></dd></dl>

<dl>
<dt id="index-TARGET_005fHARD_005fREGNO_005fMODE_005fOK">Target Hook: <em>bool</em> <strong>TARGET_HARD_REGNO_MODE_OK</strong> <em>(unsigned int <var>regno</var>, machine_mode <var>mode</var>)</em></dt>
<dd><p>This hook returns true if it is permissible to store a value
of mode <var>mode</var> in hard register number <var>regno</var> (or in several
registers starting with that one).  The default definition returns true
unconditionally.
</p>
<p>You need not include code to check for the numbers of fixed registers,
because the allocation mechanism considers them to be always occupied.
</p>
<span id="index-register-pairs"></span>
<p>On some machines, double-precision values must be kept in even/odd
register pairs.  You can implement that by defining this hook to reject
odd register numbers for such modes.
</p>
<p>The minimum requirement for a mode to be OK in a register is that the
&lsquo;<samp>mov<var>mode</var></samp>&rsquo; instruction pattern support moves between the
register and other hard register in the same class and that moving a
value into the register and back out not alter it.
</p>
<p>Since the same instruction used to move <code>word_mode</code> will work for
all narrower integer modes, it is not necessary on any machine for
this hook to distinguish between these modes, provided you define
patterns &lsquo;<samp>movhi</samp>&rsquo;, etc., to take advantage of this.  This is
useful because of the interaction between <code>TARGET_HARD_REGNO_MODE_OK</code>
and <code>TARGET_MODES_TIEABLE_P</code>; it is very desirable for all integer
modes to be tieable.
</p>
<p>Many machines have special registers for floating point arithmetic.
Often people assume that floating point machine modes are allowed only
in floating point registers.  This is not true.  Any registers that
can hold integers can safely <em>hold</em> a floating point machine
mode, whether or not floating arithmetic can be done on it in those
registers.  Integer move instructions can be used to move the values.
</p>
<p>On some machines, though, the converse is true: fixed-point machine
modes may not go in floating registers.  This is true if the floating
registers normalize any value stored in them, because storing a
non-floating value there would garble it.  In this case,
<code>TARGET_HARD_REGNO_MODE_OK</code> should reject fixed-point machine modes in
floating registers.  But if the floating registers do not automatically
normalize, if you can store any bit pattern in one and retrieve it
unchanged without a trap, then any machine mode may go in a floating
register, so you can define this hook to say so.
</p>
<p>The primary significance of special floating registers is rather that
they are the registers acceptable in floating point arithmetic
instructions.  However, this is of no concern to
<code>TARGET_HARD_REGNO_MODE_OK</code>.  You handle it by writing the proper
constraints for those instructions.
</p>
<p>On some machines, the floating registers are especially slow to access,
so that it is better to store a value in a stack frame than in such a
register if floating point arithmetic is not being done.  As long as the
floating registers are not in class <code>GENERAL_REGS</code>, they will not
be used unless some pattern&rsquo;s constraint asks for one.
</p></dd></dl>

<dl>
<dt id="index-HARD_005fREGNO_005fRENAME_005fOK">Macro: <strong>HARD_REGNO_RENAME_OK</strong> <em>(<var>from</var>, <var>to</var>)</em></dt>
<dd><p>A C expression that is nonzero if it is OK to rename a hard register
<var>from</var> to another hard register <var>to</var>.
</p>
<p>One common use of this macro is to prevent renaming of a register to
another register that is not saved by a prologue in an interrupt
handler.
</p>
<p>The default is always nonzero.
</p></dd></dl>

<dl>
<dt id="index-TARGET_005fMODES_005fTIEABLE_005fP">Target Hook: <em>bool</em> <strong>TARGET_MODES_TIEABLE_P</strong> <em>(machine_mode <var>mode1</var>, machine_mode <var>mode2</var>)</em></dt>
<dd><p>This hook returns true if a value of mode <var>mode1</var> is accessible
in mode <var>mode2</var> without copying.
</p>
<p>If <code>TARGET_HARD_REGNO_MODE_OK (<var>r</var>, <var>mode1</var>)</code> and
<code>TARGET_HARD_REGNO_MODE_OK (<var>r</var>, <var>mode2</var>)</code> are always
the same for any <var>r</var>, then
<code>TARGET_MODES_TIEABLE_P (<var>mode1</var>, <var>mode2</var>)</code>
should be true.  If they differ for any <var>r</var>, you should define
this hook to return false unless some other mechanism ensures the
accessibility of the value in a narrower mode.
</p>
<p>You should define this hook to return true in as many cases as
possible since doing so will allow GCC to perform better register
allocation.  The default definition returns true unconditionally.
</p></dd></dl>

<dl>
<dt id="index-TARGET_005fHARD_005fREGNO_005fSCRATCH_005fOK">Target Hook: <em>bool</em> <strong>TARGET_HARD_REGNO_SCRATCH_OK</strong> <em>(unsigned int <var>regno</var>)</em></dt>
<dd><p>This target hook should return <code>true</code> if it is OK to use a hard register
<var>regno</var> as scratch reg in peephole2.
</p>
<p>One common use of this macro is to prevent using of a register that
is not saved by a prologue in an interrupt handler.
</p>
<p>The default version of this hook always returns <code>true</code>.
</p></dd></dl>

<dl>
<dt id="index-AVOID_005fCCMODE_005fCOPIES">Macro: <strong>AVOID_CCMODE_COPIES</strong></dt>
<dd><p>Define this macro if the compiler should avoid copies to/from <code>CCmode</code>
registers.  You should only define this macro if support for copying to/from
<code>CCmode</code> is incomplete.
</p></dd></dl>

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